CS162 Operating Systems and Systems Programming Lecture 10 Deadlock cont d Thread Scheduling September 30 2009 Prof John Kubiatowicz http inst eecs berkeley edu cs162 Review Deadlock Starvation vs Deadlock Starvation thread waits indefinitely Deadlock circular waiting for resources Deadlock Starvation but not other way around Four conditions for deadlocks Mutual exclusion Only one thread at a time can use a resource Hold and wait Thread holding at least one resource is waiting to acquire additional resources held by other threads No preemption Resources are released only voluntarily by the threads Circular wait There exists a set T1 Tn of threads with a cyclic waiting pattern 9 30 09 Kubiatowicz CS162 UCB Fall 2009 Lec 10 2 Review Resource Allocation Graph Examples Recall request edge directed edge T1 Rj assignment edge directed edge Rj Ti R1 T1 R2 T2 R3 R1 T3 R4 Simple Resource Allocation Graph 9 30 09 T1 R2 T2 R3 R1 T3 R4 Allocation Graph With Deadlock Kubiatowicz CS162 UCB Fall 2009 T1 T2 T3 R2 T4 Allocation Graph With Cycle but No Deadlock Lec 10 3 Review Methods for Handling Deadlocks Allow system to enter deadlock and then recover Requires deadlock detection algorithm Some technique for selectively preempting resources and or terminating tasks Ensure that system will never enter a deadlock Need to monitor all lock acquisitions Selectively deny those that might lead to deadlock Ignore the problem and pretend that deadlocks never occur in the system used by most operating systems including UNIX 9 30 09 Kubiatowicz CS162 UCB Fall 2009 Lec 10 4 Goals for Today Preventing Deadlock Scheduling Policy goals Policy Options Implementation Considerations Note Some slides and or pictures in the following are adapted from slides 2005 Silberschatz Galvin and 9 30 09 Kubiatowicz CS162 UCB Fall 2009 Lec 10 5 Gagne Many slides Gagne generated from my lecture notes Deadlock Detection Algorithm Only one of each type of resource look for loops More General Deadlock Detection Algorithm Let X represent an m ary vector of non negative integers quantities of resources of each type FreeResources RequestX AllocX Current free resources each type Current requests from thread X Current resources held by thread X See if tasks can eventually terminate on their own Avail FreeResources Add all nodes to UNFINISHED do done true Foreach node in UNFINISHED if Requestnode Avail remove node from UNFINISHED Avail Avail Allocnode done false until done R1 T1 Nodes left in UNFINISHED deadlocked 9 30 09 Kubiatowicz CS162 UCB Fall 2009 T2 T3 R2 T4 Lec 10 6 What to do when detect deadlock Terminate thread force it to give up resources In Bridge example Godzilla picks up a car hurls it into the river Deadlock solved Shoot a dining lawyer But not always possible killing a thread holding a mutex leaves world inconsistent Preempt resources without killing off thread Take away resources from thread temporarily Doesn t always fit with semantics of computation Roll back actions of deadlocked threads Hit the rewind button on TiVo pretend last few minutes never happened For bridge example make one car roll backwards may require others behind him Common technique in databases transactions Of course if you restart in exactly the same way may reenter deadlock once again Many operating systems use other options 9 30 09 Kubiatowicz CS162 UCB Fall 2009 Lec 10 7 Techniques for Preventing Deadlock Infinite resources Include enough resources so that no one ever runs out of resources Doesn t have to be infinite just large Give illusion of infinite resources e g virtual memory Examples Bay bridge with 12 000 lanes Never wait Infinite disk space not realistic yet No Sharing of resources totally independent threads Not very realistic Don t allow waiting How the phone company avoids deadlock Call to your Mom in Toledo works its way through the phone lines but if blocked get busy signal Technique used in Ethernet some multiprocessor nets Everyone speaks at once On collision back off and retry Inefficient since have to keep retrying 9 30 09 Consider driving to San Francisco when hit traffic jam suddenly you re transported back home and told to retry Kubiatowicz CS162 UCB Fall 2009 Lec 10 8 Techniques for Preventing Deadlock con t Make all threads request everything they ll need at the beginning Problem Predicting future is hard tend to overestimate resources Example If need 2 chopsticks request both at same time Don t leave home until we know no one is using any intersection between here and where you want to go only one car on the Bay Bridge at a time Force all threads to request resources in a particular order preventing any cyclic use of resources Thus preventing deadlock Example x P y P z P 9 30 09 Make tasks request disk then memory then Keep from deadlock on freeways around SF by UCB Fall 2009 Lec 10 9 requiring Kubiatowicz everyoneCS162 to go clockwise Review Train Example Wormhole Routed Network Circular dependency Deadlock Each train wants to turn right Blocked by other trains Similar problem to multiprocessor networks Fix Imagine grid extends in all four directions Force ordering of channels tracks Protocol Always go east west first then north south Called dimension ordering X then Y ed w lo le al Ru is D By 9 30 09 Kubiatowicz CS162 UCB Fall 2009 Lec 10 10 Banker s Algorithm for Preventing Deadlock Toward right idea State maximum resource needs in advance Allow particular thread to proceed if available resources requested max remaining that might be needed by any thread Banker s algorithm less conservative Allocate resources dynamically Evaluate each request and grant if some ordering of threads is still deadlock free afterward Technique pretend each request is granted then run deadlock detection algorithm substituting Maxnode Allocnode Avail for Request node Avail Grant request if result is deadlock free conservative Keeps system in a SAFE state i e there exists a sequence T1 T2 Tn with T1 requesting all remaining resources finishing then T2 requesting all remaining resources etc Algorithm allows the sum of maximum resource needs of all current threads to be greater than total 9 30 09 Kubiatowicz CS162 UCB Fall 2009 Lec 10 11 resources Banker s Algorithm Example Banker s algorithm with dining lawyers Safe won t cause deadlock if when try to grab chopstick either Not last chopstick Is last chopstick but someone will have two afterwards What if k handed lawyers Don t allow if 9 30 09 It s the last one no one would have k It s 2nd
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